Lighting Device

ABSTRACT

One object is to provide a lighting device having a large irradiation range at low cost. One object is to provide a lighting device with improved light extraction efficiency at low cost. The lighting device includes a light-transmitting base, a first light-transmitting electrode formed over almost the whole area of a surface of the light-transmitting base, an EL layer over the first light-transmitting electrode, and a second electrode over the EL layer. The light-transmitting base has a cylindrical shape, a conical shape, a prismatic shape, or a pyramidal shape whose bottom surface is the surface of the light-transmitting base.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An embodiment of the invention relates to a lighting device. Inparticular, an embodiment of the invention relates to a lighting deviceutilizing electroluminescence.

2. Description of the Related Art

In recent years, research and development have been actively conductedon light-emitting elements utilizing electroluminescence. A basicstructure of a light-emitting element utilizing electroluminescence isthat in which a layer including a light-emitting substance is sandwichedbetween a pair of electrodes. By applying voltage between the electrodesof the light-emitting element, light from the light-emitting substancecan be obtained.

A light-emitting element utilizing electroluminescence includes, in itscategory, a light-emitting element using a light-emitting substance ofan organic compound and that using a light-emitting substance of aninorganic compound. In particular, application of the light-emittingelement using a light-emitting substance of an organic compound to aplanar light source is expected because the light-emitting element iseasily enlarged due to its manufacturing method.

As examples of the applications of a planar light source, a displaydevice, a backlight of a display device, a lighting device, and the likeare given. Since light extraction efficiency (luminance) is emphasizedin any of these applications, various techniques have been proposed sofar to improve light extraction efficiency.

One of these techniques is a method in which a three-dimensionalminiaturized structure is formed over a glass substrate and the totalreflection component at a surface is reduced, so that light extractionefficiency is improved (for example, see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1]-   Japanese Published Patent Application No. 2008-10245

SUMMARY OF THE INVENTION

It is certainly possible to improve light extraction efficiency from onesurface of a substrate by using the above method. However, sinceproduction cost is inevitably increased by forming a miniaturizedstructure, the above method is unsuitable for an inexpensive lightingdevice.

In addition, since a large irradiation region of illumination light isneeded for a lighting device, it is reasonable to improve extractionefficiency from all the surfaces (other than a surface for which alight-emitting element is formed) of a substrate as compared toimprovement of extraction efficiency of only one surface of thesubstrate.

In view of the above problems, one object of an embodiment of thedisclosed invention is to provide a lighting device having a largeirradiation range at low cost. One object is to provide a lightingdevice with improved light extraction efficiency at low cost.

In an embodiment of the disclosed invention in this specification andthe like, by using a light-transmitting base having a predeterminedshape as a support of a light-emitting element, a large irradiationrange and high extraction efficiency are obtained. Details thereof aredescribed below.

An embodiment of the disclosed invention is a lighting device includinga light-transmitting base, a first light-transmitting electrode formedover a surface (more preferably, almost the whole area of the surface)of the light-transmitting base, an electroluminescence (EL) layer overthe first light-transmitting electrode, and a second electrode over theEL layer. The light-transmitting base has a cylindrical shape, a conicalshape, a prismatic shape, or a pyramidal shape whose bottom surface isthe surface of the light-transmitting base.

Another embodiment of the disclosed invention is a lighting deviceincluding a light-transmitting substrate, a first light-transmittingelectrode formed over a surface (more preferably, almost the whole areaof the surface) of the light-transmitting substrate, an EL layer overthe first light-transmitting electrode, a second electrode over the ELlayer, and a light-transmitting base attached to a surface opposite tothe surface of the light-transmitting substrate. The light-transmittingbase has a cylindrical shape, a conical shape, a prismatic shape, or apyramidal shape whose bottom surface is the surface attached to thelight-transmitting substrate.

In the above, it is preferable that the light-transmitting base have aprismatic shape with a bottom surface having m angles (4≦m≦∞), and thefollowing formula as for relation of a height (h) of thelight-transmitting base, a refractive index (n) of a material of thelight-transmitting base, and a length (l) of a square circumscribing thebottom surface, be satisfied.

h≧(−0.75·n+1.98)·l

Here, in the case where m is infinite, the light-transmitting base canhave a cylindrical shape, and the length of one side of a squarecircumscribing the bottom surface is equal to the diameter of the bottomsurface having a round shape. In addition, in the case where m isfinite, there may be a plurality of squares circumscribing the bottomsurface having m angles in some cases, and any of these squares can be areference when the length of one side (1) is calculated.

In the above, another light-transmitting base having the same shape asthe light-transmitting base can be formed over the second electrode.

Note that the EL layer can be formed with a layer of any material aslong as the layer emits light due to its electric action and therefore,the structure and the material thereof do not need to be limited. Forexample, an organic EL layer formed mainly using an organic material oran inorganic EL layer formed mainly using an inorganic material may beused.

According to an embodiment of the disclosed invention, a lighting devicewith a large irradiation range can be obtained at low cost. In addition,a lighting device with sufficient extraction efficiency can be obtainedat low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate an example of a lighting device;

FIGS. 2A and 2B illustrate an example of a lighting device;

FIGS. 3A to 3C illustrates examples of a lighting device;

FIG. 4 illustrates a calculation model;

FIGS. 5A to 5I each illustrate a state of light emission;

FIGS. 6A to 6C each illustrate a state of light emission;

FIGS. 7A to 7C each illustrate a state of light emission;

FIGS. 8A to 8D each illustrate a state of light emission;

FIG. 9 shows relation between a lower limit (h_(Low)) of the height of abase and a refractive index (n) of the base;

FIG. 10 shows relation between a lower limit (h_(Low)) of the height ofa base and a refractive index (n) of the base;

FIGS. 11A to 11C illustrate a shape of a bottom surface of a base;

FIG. 12 illustrates an example of a lighting device;

FIG. 13 illustrates an example of a light-emitting element;

FIG. 14 illustrates an example of a light-emitting element;

FIG. 15 illustrates an example of a lighting device; and

FIG. 16 illustrates an example of a lighting device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, Embodiments are described in detail using the drawings.However, the invention is not limited to the following description, andit will be easily understood by those skilled in the art that variouschanges and modifications can be made to the modes and their detailswithout departing from the spirit of the invention. A structure of thedifferent embodiment can be implemented by combination appropriately. Onthe description of the invention with reference to the drawings, areference numeral indicating the same part is used in common throughoutdifferent drawings, and the repeated description is omitted.

Embodiment 1

In this embodiment, a lighting device which is an embodiment of thedisclosed invention is described with reference to FIGS. 1A and 1B,FIGS. 2A and 2B, and FIGS. 3A to 3C.

Examples of a lighting device are illustrated in FIGS. 1A and 1B andFIGS. 2A and 2B. FIG. 1A and FIG. 2A are cross sectional views eachillustrating a structural example of a lighting device 150. FIG. 1B andFIG. 2B each schematically illustrate a state in which a wide area isirradiated with light beams 130 from the lighting device 150.

The lighting device 150 illustrated in FIG. 1A or FIG. 2A includes alight-transmitting base 100, a first electrode 102 and a secondelectrode 104 over the light-transmitting base 100, and an EL layer 106between the first electrode 102 and the second electrode 104. In theabove, a stacked structure of the first electrode 102, the EL layer 106,and the second electrode 104 can be referred to as a light-emittingelement. Since light is extracted from the light-transmitting base 100side in the lighting device 150, the first electrode 102 needs to have alight-transmitting property. Note that a sealing material, anothersubstrate, or the like may be provided above the second electrode.

Here, it is preferable that the light-transmitting base have any of acylindrical shape, a conical shape, a prismatic shape, or a pyramidalshape whose bottom surface is a surface for which the first electrode isformed. This is because when such a shape is employed, light can beextracted from the entire surfaces other than the surface for which thefirst electrode is formed, and therefore, light extraction efficiencycan be improved. Note that FIGS. 1A and 1B illustrate the case where thelight-transmitting base has a cylindrical shape or a prismatic shape,and FIGS. 2A and 2B illustrate the case where the light-transmittingbase has a conical shape or a pyramidal shape.

FIGS. 3A to 3C are plan views each illustrating the lighting device 150of FIGS. 1A and 1B or FIGS. 2A and 2B seen from the light-emittingelement side. FIG. 3A illustrates the light-transmitting base having acylindrical shape or a conical shape. FIG. 3B illustrates thelight-transmitting base having a triangular pole shape or a triangularpyramidal shape. FIG. 3C illustrates the light-transmitting base havinga square pole shape or a square pyramidal shape. Note that the examplesin which the light-emitting element is formed over almost the entirebottom surface of the light-transmitting base are described here;however, this embodiment of the disclosed invention is not limitedthereto. The light-emitting element can be formed over only part of thebottom surface of the light-transmitting base. In this case, the planarshape of the light-emitting element may be similar to the bottom surfaceof the light-transmitting base, or may be set irrespectively of theshape of the bottom surface of the light-transmitting base.

Note that “light-transmitting” in this specification means having atransmittance of 60% or more with respect to light which has awavelength having a peak within a wavelength range of 400 nm to 800 nmin an emission spectrum of the EL layer. Further, “base” means a supportof the light-emitting element or a structure including thelight-emitting element. The “base” also means a support whose bottomsurface area (the area of one bottom surface in the case of having aplurality of bottom surfaces) accounts for less than 40% of the totalarea of the entire surfaces of the support. In addition, “bottomsurface” of the base means a surface for which the first electrode (thelight-emitting element) is formed or a surface approximately parallel tothe surface for which the first electrode (the light-emitting element)is formed.

Note that although the example in which the light-emitting element isformed on the base directly is described in this embodiment, thelight-emitting element may be formed over a light-transmitting substrateand then the light-transmitting substrate may be attached to thelight-transmitting base. Here, “substrate” means a support which is arectangular solid in which the sum of areas of one pair of surfacesfacing each other accounts for 80% or more of the total area of theentire surfaces. In other words, “substrate” refers to a support whichis a rectangular solid in which the area of the surface for which thefirst electrode (the light-emitting element) is formed accounts for 40%or more of the total area of the entire surfaces. Note that in the casewhere a refractive index differs between the substrate and the base, aloss of light due to reflection at an interface between the substrateand the base becomes a problem. Therefore, it is preferable that amaterial of the substrate and a material of the base have approximatelythe same refractive index. The same applies to a material with which thesubstrate and the base are attached to each other. It is preferable thatthe material with which the substrate and the base are attached to eachother have approximately the same refractive index as those of thesubstrate and the base.

As described in this embodiment, in the case where a light-transmittingbase is used as a support of the light-emitting element, a sufficientirradiation range can be secured without a miniaturized structure whichcauses increase in cost. Therefore, a lighting device with excellentcharacteristics can be manufactured at low cost, as compared to the casewhere a miniaturized structure is formed.

Note that although the base has any of a cylindrical shape, a conicalshape, a prismatic shape, or a pyramidal shape in the above, the basemay have a truncated conical shape, a truncated pyramidal shape, or asemispherical shape when processing steps of the base are notcomplicated. Further, corners of the base may be chamfered. That is, thebase does not necessarily have an accurate cylindrical shape, anaccurate conical shape, an accurate prismatic shape, an accuratepyramidal shape, or the like.

Embodiment 2

In this embodiment, the shape of the base for realizing a lightingdevice in which an irradiation range was secured and extractionefficiency was improved was obtained using computer simulation.Calculation results and a suitable shape of the base are described usingFIG. 4, FIGS. 5A to 5I, FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to 8D,FIG. 9, FIG. 10, and FIGS. 11A to 11C.

Note that all the calculation below is performed using Light Tools ver.6.3.0 manufactured by Optical Research Associates (ORA).

<Relation Between Size of Light-Emitting Surface and Height of Base>

First, in order to confirm relation between the size of a planar lightsource and the height (h) of the base, the state of light irradiationwas confirmed at each of a plurality of different heights (h). Here,“height” of the base refers to a length of the base in a directionapproximately perpendicular to the bottom surface of the base (see FIG.1A and the like). Here, a square planar light source (Lambert lightsource) with an emission wavelength of 550 nm and a size of 100 mm×100mm was typically used, and the light-transmitting base which had asquare pole shape with a size of 100 mm×100 mm×h mm was used. Inaddition, BK7 manufactured by Schott Inc. was used as a material of thelight-transmitting base. That is, as a calculation model, a lightingdevice similar to those in FIG. 1A and FIG. 3C was used.

FIG. 4 is a schematic view of a model used for the calculation. Thelighting device 150 is placed at the center of the calculation model.The lighting device 150 is placed so that the light-emitting element ispositioned on the upper side and the light-transmitting base ispositioned on the lower side. From the calculation, the state of thelight beam 130 emitted from the lighting device 150 to a wall surface160 surrounding the lighting device 150 is confirmed.

The respective states of light irradiation under conditions withdifferent heights (h) of the base are illustrated in FIGS. 5A to 5I. Inthe vicinity of the center of each of the drawings, the lighting deviceis placed in a similar manner to that of FIG. 4. That is, the lightingdevice is placed in order that light from the light-emitting element isextracted to the lower side and the lateral side. In addition, in thedrawings, a path of the light beam is denoted by a line segment.

Here, FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H, and 5I illustrate the caseswhere the heights (h) of the base are 0.7 mm, 2 mm, 10 mm, 30 mm, 50 mm,70 mm, 80 mm, 84 mm, and 90 mm, respectively.

From FIGS. 5A to 5I, it is found that light is emitted to the upper sideof the light-emitting element (i.e., in a direction opposite to thelight-transmitting base) in the case where the height of the base issmall. In this case, it is necessary that the height of the base is 84mm or higher in order that all the light from the light-emitting elementis emitted to the lower side and the lateral side (i.e., to a directionof the light-transmitting base).

Next, a state of light irradiation in the case where a square planarlight source (Lambert light source) with a size of 200 mm×200 mm is usedas a light source, and the light-transmitting base which has a squarepole shape with a size of 200 mm×200 mm×h mm is used is illustrated inFIGS. 6A to 6C. Note that the other conditions are the same as those ofFIGS. 5A to 5I. Here, FIGS. 6A, 6B, and 6C illustrate the cases wherethe heights (h) of the base are 160 mm, 168 mm, and 170 mm,respectively.

From FIGS. 6A to 6C, it is found that all the light is emitted to thelower side and the lateral side (i.e., to the direction of thelight-transmitting base) when the height of the base is 168 mm or largerin the case where the size of the planar light source is 200 mm×200 mm.

Next, a state of light irradiation in the case where a rectangularplanar light source (Lambert light source) with a size of 100 mm×200 mmis used as a light source and the light-transmitting base which has asquare pole shape with a size of 100 mm×200 mm×h mm is used isillustrated in FIGS. 7A to 7C. Note that in FIGS. 7A to 7C, the totalamount of the light beam is 100 times as that of FIG. 5A to 5I and thatof FIG. 6A to 6C and the other conditions are the same as those of FIG.5A to 5I and 6A to 6C. Here, FIGS. 7A, 7B, and 7C illustrate the caseswhere the heights (h) of the base are 160 mm, 168 mm, and 170 mm,respectively.

From FIGS. 7A to 7C, it is found that all the light is emitted to thelower side and the lateral side (i.e., to the direction of thelight-transmitting base) when the height of the base is 168 mm or higherin the case where the size of the planar light source is 100 mm×200 mm.

From the FIGS. 5A to 5I, FIGS. 6A to 6C, and FIGS. 7A to 7C, it can besaid that in the case where the emission wavelength is 550 nm, thelight-transmitting base has a square pole shape, and BK7 manufactured bySchott Inc. is used for the material of the light-transmitting base,when the following formula as for the relation between the height (h) ofthe base (see FIG. 1A) and the length (l₁) of a long side of the bottomsurface of the base (or the planar light source) (see FIG. 3C) issatisfied, all the light from the light-emitting element is emitted tothe lower side and the lateral side (i.e., to the direction of thelight-transmitting base), so that all the light can be effectively used.

h≧0.84·l ₁

<Relation Between Emission Wavelength and Height of Base>

Next, the state of light irradiation in the cases of the emissionwavelengths of 400 nm and 800 nm was confirmed in order to confirm therelation between the emission wavelength and the height (h) of the base.Here, the above emission wavelengths were selected because 400 nm is atypical value of a shorter wavelength of the visible range and 800 nm isa typical value of a longer wavelength of the visible range. Note thatthe conditions other than the emission wavelength and the amount oflight beam (100 times as that of FIGS. 5A to 5I) are the same as thoseof FIGS. 5A to 5I.

The state of light irradiation with the respective wavelengths areillustrated in FIGS. 8A to 8D. FIGS. 8A, 8B, 8C, and 8D illustrate thecases where the emission wavelengths and the heights (h) of the baseare: 400 nm and 80 mm, 400 nm and 81 mm, 800 nm and 83 mm, and 800 nmand 84 mm, respectively.

From FIGS. 8A to 8D, it is found that in order to effective use all thelight from the light-emitting element, the height (h) of the base needsto be 81 mm or higher in the case of the emission wavelength of 400 nm,and the height (h) of the base needs to be 84 mm in the case of theemission wavelength of 800 nm. In other words, it is understood that thelower limit (h_(Low)) of the height of the base tends to be higher onthe long wavelength side. In a lighting device for general use such as awhite emission light (other than a lighting device with a peculiaremission wavelength, such as an infrared light source), the value at thewavelength of 800 nm can represent the lower limit (h_(Low)) of theheight of the base.

When the above description is taken into the consideration, it can besaid that in the case where the light-transmitting base has a squarepole shape and BK7 manufactured by Schott Inc. is used for the materialof the light-transmitting base, when the following formula as for therelation between the height (h) of the base and the length (l₁) of thelong side of the bottom surface of the base (or the planar light source)is satisfied, all the light from the light-emitting element is emittedto the lower side and the lateral side (i.e., to the direction of thelight-transmitting base), so that all the light can be effectively used.

h≧0.84·l ₁

<Relation Between Refractive Index of Base and Height of Base>

Next, in order to confirm relation between the refractive index (n) ofthe base and the height (h) of the base, the material of thelight-transmitting base is changed and the state of light irradiation isconfirmed. Here, an emission wavelength of 800 nm is selected, and PK51manufactured by Schott Inc. (the refractive index is 1.52 at an emissionwavelength of 800 nm), BAK4 manufactured by Schott Inc. (the refractiveindex is 1.56 at an emission wavelength of 800 nm), and polycarbonate(the refractive index is 1.57 at an emission wavelength of 800 nm) areselected for the material of the light-transmitting base. Note that theconditions other than the emission wavelength, the material of the base(refractive index), and the amount of light beam (100 times as that ofFIGS. 5A to 5I) are the same as those of FIGS. 5A to 5I.

FIG. 9 shows the relation between the refractive index (n) of the baseand the lower limit (h_(Low)) of the height of the base, which isderived from the above results, for the purpose of effective use of allthe light from the light-emitting element. In FIG. 9, the horizontalaxis represents the refractive index (n) and the vertical axisrepresents the lower limit (h_(Low)) of the height of the base. Inaddition, a straight line in FIG. 9 links the result of the PK51 (therefractive index: 1.52) and the result of the BAK4 (the refractiveindex: 1.56). Although the results of the polycarbonate (the refractiveindex: 1.57) slightly deviates from the straight line, the straight lineis regarded as the lower limit (h_(Low)) of the height of the base.

Accordingly, it is understood that generally, the relation between therefractive index (n) and the lower limit (h_(Low)) of the height of thebase is as follows:

h _(Low)=−75·n+198(mm)

Considering that h_(Low) is the lower limit of the height (h) in thecase where the long side of the planar light source is 100 mm, the lowerlimit (h_(Low, 1)) of the height (h) of the base in the case where thelong side of the bottom surface of the base (or the planar light source)is 1 mm is in the following relation:

−75·n+198:100=h _(Low, 1) :l, that is,

h _(Low, 1)=(−0.75·n+1.98)·l

Accordingly, in the case of the light-transmitting base having a squarepole shape, when the following formula as for the height (h) of the baseis satisfied, all the light from the light-emitting element is emittedto the lower side and the lateral side (i.e., to the direction of thelight-transmitting base), so that all the light can be effectively used.

h≧(−0.75·n+1.98)·l

<Relation Between Shape of Bottom Surface of Base (Planar Light Source)and Height of Base>

Next, in order to confirm relation between the shape of the bottomsurface of the base (or the planar light source) and the height (h) ofthe base, the same calculation as that of <Relation between refractiveindex of base and height of base> is performed in the case of acylindrical shape. Specifically, in <Relation between refractive indexof base and height of base>, the shape of the planar light source ischange to a round shape having a diameter of 100 mm and the shape of thelight-transmitting base is change to a cylindrical shape with a bottomsurface having a diameter of 100 mm and a height of h mm, and then astate of light irradiation is confirmed.

FIG. 10 shows the relation between the refractive index (n) of the baseand the lower limit (h_(Low)) of the height of the base, which isderived from the above results, for the purpose of effective use of allthe light from the light-emitting element. In FIG. 10, the horizontalaxis represents the refractive index (n) and the vertical axisrepresents the lower limit (h_(Low)) of the height of the base. Inaddition, a straight line in FIG. 10 links the result of the PK51 (therefractive index: 1.52) and the result of the BAK4 (the refractiveindex: 1.56). Although the result of the polycarbonate (the refractiveindex: 1.57) slightly deviates from the straight line, in a similarmanner to the case of FIG. 9, the straight line is regarded as the lowerlimit (h_(Low)) of the height of the base.

Accordingly, it is understood that generally, the relation between therefractive index (n) and the lower limit (h_(Low)) of the height of thebase is as follows:

h _(Low)=−100·n+234(mm)

Accordingly, in the case of the light-transmitting base having acylindrical shape, when the following formula as for the height (h) ofthe base is satisfied, all the light from the light-emitting element isemitted to the lower side and the lateral side (i.e., to the directionof the light-transmitting base), so that all the light can beeffectively used.

h≧(−n+2.34)·l

Note that in the above formula, 1 mm is the diameter of the bottomsurface of the base (or the planar light source).

Note that in the case of a general material, the above conditions aremore lenient than the conditions described in <Relation betweenrefractive index of base and height of base>. Therefore, in the casewhere the light-transmitting base has a cylindrical shape, the lowerlimit of the height (h) can be estimated with reference to theconditions described in <Relation between the refractive index of thebase and the height of the base>. Specifically, for example, a squarepole shape whose bottom surface is a square circumscribing the bottomsurface can be used as a reference for estimation. In other words, inthe case of the length (l) of one side of the square circumscribing thebottom surface, when the following formula as for the height (h) of thebase is satisfied, all the light from the light-emitting element isemitted to the lower side and the lateral side (i.e., to the directionof the light-transmitting base), so that all the light can beeffectively used.

h≧(−0.75·n+1.98)·l

FIGS. 11A and 11B schematically illustrate the case where the base 100has a cylindrical shape. As illustrated in FIG. 11B, assuming thatexistence of a square 170 tangent to the bottom surface of the base 100from the outside, the length (l) of one side of the square 170 can beused as a reference for estimation.

The same applies to the case where the base has a prismatic shape with abottom surface which is a polygon having m angles (4≦m≦∞). This isbecause as the number of m is increased, the shape of the base becomescloser to a cylindrical shape and therefore, the lower limit of theheight (h) has an intermediate value between the lower limit of theheight (h) in the case of a square pole shape and that in the case of acylindrical shape.

The summary of the above is as follows. In the case where thelight-transmitting base has a prismatic shape with a bottom surfacehaving m angles (4≦m≦∞), when the following formula as for relation ofthe height (h), the refractive index (n) of the material of the base,and the length (l) of one surface of a square circumscribing the bottomsurface (tangent to the bottom surface from the outside) is satisfied,all the light from the light-emitting element is emitted to the lowerside and the lateral side (i.e., to the direction of thelight-transmitting base), so that all the light can be effectively used.

h≧(−0.75·n+1.98)·l

Here, “m=∞” means to the case where the shape of the light-transmittingbase has a cylindrical shape. In addition, although there may be aplurality of squares circumscribing the bottom surface having m anglesin the case where m is finite, when the length (l) of one side iscalculated, any of the squares can be used as a reference.

For example, in the case where the base 100 has a prismatic shape with aheptangular bottom surface, as illustrated in FIG. 11C, assumingexistence of the square 170 (preferably a square having the minimumarea) tangent to the heptangular bottom surface, the length (l) of oneside of the square 170 can be used as a reference for estimation.

When the base with which the above conditions are satisfied is used, allthe light can be effectively used. Accordingly, a lighting device inwhich an irradiation range is secured and light extraction efficiency isimproved can be obtained.

Embodiment 3

In this embodiment, a modified example of a lighting device which is anembodiment of the disclosed invention is described using FIG. 12.

FIG. 12 illustrates a lighting device in which anotherlight-transmitting base having the same shape as the light-transmittingbase is provided above the second electrode of the lighting devicedescribed in the aforementioned embodiment (on a side opposite to thelight transmitting base).

With such a structure, a lighting device which can emit light from alight-emitting element to all directions can be obtained. In thelighting device described in this embodiment, a dual-emission-typelight-emitting element is used.

Note that the structure described this embodiment can be combined asappropriate with any structure described in the other embodiments.

Embodiment 4

In this embodiment, details of a lighting device and a light-emittingelement used for the lighting device are described using FIG. 13. Notethat although an organic EL element formed mainly using an organicmaterial is used as the light-emitting element as an example, aninorganic EL element formed using an inorganic material can also beused.

<Base>

A light-transmitting base 100 functions as a support of thelight-emitting element. The light-transmitting base 100 can be formedusing, for example, an insulating material such as glass or plastic.Note that a base formed using a material other than the above materialmay be used as long as the light-transmitting property of the base issecured.

Although the shape of the light-transmitting base 100 is as described inthe above embodiment, processing for making the light-transmitting base100 to have the shape can be performed either before formation of thelight-emitting element or after formation of the light-emitting element.Needless to say, the base may be processed to have a desired shape at astage of formation of the base.

<First electrode>

A first electrode 102 functions as an anode and as a light-extractionelectrode. It is preferable that a material having a large work functionbe used for the first electrode 102 used as an anode. Specifically,conductive oxide such as indium tin oxide (ITO), indium tin oxidecontaining silicon oxide (ITSO), and indium oxide containing zinc oxide(IZO) at 2 wt % to 20 wt % can be used.

Note that the above conductive oxide is usually formed by a sputteringmethod, but may also be formed by an inkjet method, a spin coatingmethod, or the like by application of sol-gel method or the like. When asputtering method is used, for example, indium oxide and zinc oxide(IZO) can be formed using indium oxide to which zinc oxide is added at 1to 20 wt % as a target. Indium oxide containing tungsten oxide and zincoxide can be formed by sputtering using a target in which tungsten oxideand zinc oxide are added to indium oxide at 0.5 wt % to 5 wt % and 0.1wt % to 1 wt %, respectively.

In addition, a material such as gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), or palladium (Pd) may be used. In this case, in order tosecure a predetermined transmittance, it is preferable that the firstelectrode 102 be thinly formed.

Note that in order to effectively extract light generated in an EL layerto the outside, it is preferable that transmittance of the firstelectrode 102 at a visible light range (a wavelength range of greaterthan or equal to 400 nm and less than or equal to 800 nm) be 70% ormore.

<EL Layer>

An EL layer 106 includes at least a light-emitting layer 114. In thelight-emitting element described in this embodiment, the EL layer 106includes a layer 110 including a composite material, a hole-transportlayer 112, the light-emitting layer 114, an electron-transport layer116, and an electron-injection layer 118; however, the structures otherthan the light-emitting layer 114 are optional. In other words, the ELlayer 106 can be formed with an appropriate combination of a layerincluding a substance having a high electron-transport property, asubstance having a high hole-transport property, a substance having ahigh electron-injection property, a substance having a highhole-injection property, a bipolar substance (a substance having a highelectron-transport property and a high hole-transport property), or thelike; a light-emitting layer; a layer including a composite material,and the like.

Any of various methods can be employed for forming the EL layer 106regardless of whether it is a dry process or a wet process. For example,a vacuum evaporation method, an inkjet method, a spin coating method, orthe like can be used. As described above, the EL layer 106 is formedwith a stacked structure of the layer including a composite material,the hole-transport layer, the light-emitting layer, theelectron-transport layer, the electron-injection layer (a buffer layer),and the like, and these layers are formed using a common depositionmethod, so that simplification of a process or the like can beperformed. Details of these layers are described below.

<EL Layer—Layer Including Composite Material>

The layer 110 including a composite material is a layer including acomposite material in which an acceptor substance is added to asubstance having a high hole-transport property. By using such acomposite material, an excellent hole-injection property from the firstelectrode 102 can be obtained. The layer 110 including a compositematerial can be formed by co-evaporation of a substance having a highhole-transport property and an acceptor substance, for example.

Although there is no particular limitation on the substance having ahigh hole-transport property as long as the substance having a highhole-transport property has a hole-transport property higher than anelectron-transport property, a substance having hole mobility of 10⁻⁶cm²/Vs or higher is more preferable.

As examples of the organic compound having a high hole-transportproperty, for example, the following aromatic amine compounds can begiven: 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl(abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and the like.

In addition, the following carbazole derivatives can be given asexamples: 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Furthermore, the following aromatic hydrocarbon compounds can be givenas examples: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butyl-anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracen,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

Furthermore, the following high molecular compounds (oligomers,dendrimers, polymers, and the like) are given as examples:poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA),poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD), and the like.

As examples of the acceptor substance, organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbrev.: F₄-TCNQ)and chloranil and a transition metal oxide can be given. In particular,an oxide of a metal belonging to any of Group 4 to Group 8 in theperiodic table is preferably used. Specifically, vanadium oxide, niobiumoxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide having high electron-acceptingproperties, and the like are preferable. Among them, molybdenum oxide isparticularly preferable because it is stable in the atmosphere, low inhygroscopicity, and is easy to be handled.

<EL Layer—Hole-Transport Layer>

The hole-transport layer 112 is a layer that contains a substance havinga high hole-transport property. Although there is no particularlimitation on the substance having a high hole-transport property aslong as the substance having a high hole-transport property has ahole-transport property higher than an electron-transport property, asubstance having hole mobility of 10⁻⁶ cm²/Vs or higher is morepreferable.

As the substance having a high hole-transport property, the followingaromatic amine compounds are given as examples: NPB, TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and the like.

Furtherer, high molecular compounds such as PVK, PVTPA, PTPDMA, andPoly-TPD are given.

Alternatively, a composite material in which an acceptor substance iscontained in the substance having a high hole-transport property can beused.

Alternatively, the hole-transport property may be adjusted by adding anorganic compound having a hole-trapping property, a substance having ahigh electron-transport property, or a hole-blocking material to thehole-transport layer 112. The organic compound having a hole-trappingproperty preferably has an ionization potential lower than a substancehaving a high hole-transport property which is included in thehole-transport layer 112 by 0.3 eV or higher. In addition, as thesubstance having a high electron-transport property, the later-givensubstance that can be used for the electron-transport layer 116 or thelike can be used. Further, for the hole-blocking material, a materialhaving an ionization potential of 5.8 eV or higher, or a material havingan ionization potential higher than a substance having a highhole-transport property which is included in the hole-transport layer by0.5 eV or higher is preferably used. Note that the organic compoundhaving a hole-trapping property or the substance having a highelectron-transport property which is added may emit light the color ofwhich is preferably similar to emission light of the light-emittinglayer 114 in view of keeping of excellent color purity.

Note that the hole-transport layer 112 is not limited to a single layerand may have a stacked structure of two or more layers.

<EL Layer—Light-Emitting Layer>

The light-emitting layer 114 is a layer containing a substance having ahigh light-emitting property, and can be formed using various materials.As the substance having a high light-emitting property, a fluorescentcompound which emits fluorescence or a phosphorescent compound whichemits phosphorescence can be used, for example. Since a phosphorescentcompound which emits phosphorescence has a high light-emitting property,in the case where it is used for the light-emitting layer 114, anadvantage of lower power consumption and the like can be obtained.

As examples of the phosphorescent compound, the following materials forblue light emission are given:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis[2-(3′,5′bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)), and the like.

As examples of a material for green light emission,tris(2-phenylpyridinato-N,C²′)iridium(III) (abbreviation: Ir(ppy)₃),bis[2-phenylpyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation:Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)), and the like are given. As examples of a material foryellow light emission,bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)), bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(bt)₂(acac)), andthe like are given.

As a material for orange light emission,tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)), and the like are given.

Further, as examples of a material for red light emission, the followingorganometallic complexes can be given:bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)), and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP), and the like.

As examples of the fluorescent compound, the following materials forblue light emission can be given:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like

As examples of a material for green light emission,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviated to 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracene-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like are given.

As examples of a material for yellow light emission, rubrene;5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT)and the like are given.

Furthermore, as examples of a material for red light emission,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2), and the like are given.

Note that the light-emitting layer 114 may have a structure in which asubstance having a high light-emitting property (guest material) isdispersed into another substance (host material). A light-emittingsubstance (host material) can be dispersed in various kinds ofsubstances, and it is preferably dispersed in a substance that has alowest unoccupied molecular orbital (LUMO) level higher than that of thelight-emitting substance and has a highest occupied molecular orbital(HOMO) level lower than that of the light-emitting substance.

As examples of the substance in which the light-emitting substance isdispersed, the following can be given: a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂); aheterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (BCP); a condensed aromatic compound such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3),9,10-diphenylanthracene (abbreviation: DPAnth), or6,12-dimethoxy-5,11-diphenylchrysene; an aromatic amine compound such asN,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzAlPA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-antluyl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB; and thelike.

As the substance in which the light-emitting substance is dispersed, aplurality of kinds of substances may be used. For example, a substancewhich inhibits crystallization, such as rubrene, can be added to thesubstance in which the light-emitting substance is dispersed. In orderto effectively perform energy transfer to the light-emitting substance,NPB or Alq may be added.

Thus, with a structure in which a substance having a high light-emittingproperty is dispersed in another substance, crystallization of thelight-emitting layer 114 can be suppressed. Further, concentrationquenching due to high concentration of the substance having a highlight-emitting property can be suppressed.

Note that the light-emitting layer 114 is not limited to a single layerand may have a stacked structure of two or more layers.

<EL Layer—Electron Transport Layer>

The electron-transport layer 116 is a layer containing a substancehaving a high electron-transport property. Although there is noparticular limitation on the substance having a high electron-transportproperty as long as an electron-transport property thereof is higherthan a hole-transport property thereof, mainly, a substance having anelectron mobility of 10⁻⁶ cm²/Vs or higher is more preferable.

As examples of the substance having a high electron-transport property,the following metal complexes can be given:tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO),bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ), andthe like.

In addition, the following heterocyclic compound can be given asexamples: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and the like.

Further, the following high molecular compounds can also be given asexamples: poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy), and the like.

Further, by addition of an organic compound having an electron-trappingproperty or a substance having a high hole-transport property to theelectron-transport layer 116, the electron-transport property may becontrolled. As the organic compound having an electron-trappingproperty, an organic compound having an electron affinity larger thanthe substance having a high electron-transport property which isincluded in the electron-transport layer 116 by 0.30 eV or higher ispreferably used. In addition, as the substance having a highhole-transport property, a substance which can be used for thehole-transport layer 112 or the like can be used. Note that the organiccompound having an electron-trapping property and the substance having ahigh hole-transport property, which are added, may emit light the colorof which is preferably similar to the emission color of thelight-emitting layer 114 in view of keeping of excellent color purity.

Note that the electron-transport layer 116 is not limited to a singlelayer and may be have a stacked layer of two or more layers.

<EL Layer—Electron-Injection Layer>

The electron-injection layer 118 (also referred to as a buffer layer) isa layer including a substance having a high electron-injection property.

As examples of the substance having a high electron-injection property,the following alkali metals, alkaline earth metals, rare earth metals,and compounds thereof can be given: lithium (Li), cesium (Cs), magnesium(Mg), calcium (Ca), ytterbium (Yb), lithium fluoride (LiF), calciumfluoride (CaF₂), cesium fluoride (CsF), magnesium fluoride (MgF₂),lithium carbonate (Li₂CO₃), cesium carbonate (Cs₂CO₃), lithium oxide(Li₂O), erbium fluoride (ErF₃), lithium acetylacetonate,8-quinolinolato-lithium, and the like. In particular, it is preferableto use a lithium compound such as lithium fluoride (LiF), lithium oxide(Li₂O), lithium carbonate (Li₂CO₃), lithium acetylacetonate, or8-quinolinolato-lithium because of their excellent electron-injectionproperties.

Further, the electron-injection layer 118 may include a material of asubstance having an electron-transport property to which a donorsubstance is added. As the donor substance, an alkali metal, an alkalineearth metal, a rare earth metal, or a compound thereof is given. Forexample, a material of Alq to which magnesium (Mg) is added, a materialof Alq to which lithium (Li) is added, or the like can be used.

Note that the electron-injection layer 118 is not limited to a singlelayer and may have a stacked structure of two or more layers.

<Second Electrode>

The second electrode 104 functions as a cathode and as alight-reflective electrode. The second electrode 104 serving as acathode is formed using a substance having a small work function.Specifically, it is possible to use aluminum (Al), indium (In), analkali metal such as lithium (Li) or cesium (Cs), an alkaline-earthmetal such as magnesium (Mg) or calcium (Ca), a rare-earth metal such aserbium (Er) or ytterbium (Yb), or the like. The electrode can also beformed from an alloy such as aluminum-lithium alloy (AlLi) ormagnesium-silver alloy (MgAg). The second electrode 104 can be formedusing a vacuum evaporation method, a sputtering method, or the like.

Note that in the case where a dual-emission type light-emitting elementis needed as the lighting device illustrated in FIG. 12, the secondelectrode 104 is formed so as to have a light-transmitting property. Forexample, a stacked structure of a conductive layer formed using theabove material and conductive oxide used for the first electrode 102 canbe employed.

In the case where a dual-emission-type light-emitting element is formed,it is preferable that the transmittance of the second electrode 104 at avisible light range (a wavelength range of greater than or equal to 400nm and less than or equal to 800 nm) be 70% or more.

In addition, in the case where a dual-emission-type light-emittingelement is formed, a stacked structure in which the second electrode104, the electron-injection layer 118, the electron-transport layer 116,the light-emitting layer 114, the hole-transport layer 112, the layer110 including a composite material, and the first electrode 102 areprovided in this order from the base side may be employed.

Note that the structure of this embodiment can be combined asappropriate with any of structures described in the other embodiments.

Embodiment 5

In this embodiment, an example of a light-emitting element in which aplurality of light-emitting units are stacked (hereinafter thislight-emitting element is referred to as a stacked-type light-emittingelement) will be described with reference to FIG. 14.

A light-emitting element illustrated in FIG. 14 includes a firstelectrode 202, a second electrode 204, a first EL layer 206 a and asecond EL layer 206 b between the first electrode 202 and the secondelectrode 204, and a charge generation layer 208 between the first ELlayer 206 a and the second EL layer 206 b. Here, the structures of thefirst electrode 202 and the second electrode 204 are the same as thoseof the first electrode 202 and the second electrode 204 of Embodiment 1.In addition, each of the structures of the EL layer 206 a and the ELlayer 206 b is the same as the structure of the EL layer 106 describedin the above embodiments. Note that the EL layer 206 a and the EL layer206 b may have either the same structure or different structures.

The charge generation layer 208 has a function of injecting electronsinto one of the EL layers and injecting holes into the other of the ELlayers when voltage is applied between the first electrode 202 and thesecond electrode 204. Note that the charge generation layer 208 may havea single-layer structure or a stacked structure. In the case of astacked structure, for example, a structure in which a layer having afunction of injecting electrons and a layer having a function ofinjecting holes are stacked can be employed. In addition, a layerincluded in the EL layer may be combined with the charge generationlayer 208.

As the layer that injects electrons, a layer formed from a semiconductoror an insulator, such as lithium oxide, lithium fluoride, or cesiumcarbonate, can be used. Alternatively, a layer formed from a material ofa substance having a high electron-transport property to which a donorsubstance is added can be used. As the donor substance, an alkali metal,an alkaline-earth metal, a rare earth metal, a metal belonging to Group13 of the periodic table, an oxide or carbonate of any of these, or thelike can be used. For example, lithium (Li), cesium (Cs), magnesium(Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesiumcarbonate, or the like is preferably used. Alternatively, an organiccompound such as tetrathianaphthacene may be used as the donorsubstance. As the substance having a high electron-transport property,the materials described in the above embodiment can be used. Inaddition, as the substance having a high electron-transport property, asubstance having an electron mobility of 10⁻⁶ cm²/Vs or higher ispreferably used, although other substances may also be used as long asthe electron-transport property thereof is higher than thehole-transport property thereof. The composite material including thesubstance having a high electron-transport property and the donorsubstance is excellent in a carrier-injection property and acarrier-transport property, and therefore, by using the compositematerial, low-voltage driving and low-current driving can be realized.

As the hole-injection layer, a layer formed from a semiconductor or aninsulator, such as molybdenum oxide, vanadium oxide, rhenium oxide, orruthenium oxide, can be used. Alternatively, a layer formed from amaterial of a substance having a high hole-transport property to whichan acceptor substance is added may be used. The layer including asubstance having a high hole-transport property and an acceptorsubstance is formed using the composite material described in the aboveembodiment and includes, as the acceptor substance,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) or metal oxide such as vanadium oxide, molybdenum oxide, ortungsten oxide. As the substance having a high hole-transport property,various compounds such as an aromatic amine compound, a carbazolederivative, aromatic hydrocarbon, and a high-molecular compound (such asan oligomer, a dendrimer, or a polymer) can be used. In addition,although a substance having an hole mobility of 10⁻⁶ cm²/Vs or higher ispreferably used for the substance having a high hole-transport property,other substances may also be used as long as the hole-transport propertythereof is higher than the electron-transport property thereof. Thecomposite material including the substance having a high hole-transportproperty and the acceptor substance is excellent in a carrier-injectionproperty and a carrier-transport property, and therefore, by using thecomposite material, low-voltage driving and low-current driving can berealized.

Further, electrode materials described in the above embodiment can alsobe used for the charge generation layer 208. For example, the chargegeneration layer 208 may be formed by combining a layer including asubstance having a high hole-transport property and metal oxide with atransparent conductive film. Note that in view of light extractionefficiency, the charge generation layer 208 is preferably formed using alight-transmitting material.

Although the light-emitting element having the two EL layers isdescribed in this embodiment, a light-emitting element having three ormore EL layers can be employed. In that case, it is preferable that aplurality of EL layers be connected to each other with a chargegeneration layer therebetween. By providing a charge generation layerbetween EL layers, luminance can be improved while low current densityis kept, and the lifetime of the element can be prolonged.

Further, by forming the EL layers to emit light of different colors fromeach other, an emission color that is provided by the light-emittingelement as a whole can be controlled. For example, in the light-emittingelement having two EL layers, when an emission color of the first ELlayer and an emission color of the second EL layer are made to becomplementary colors, it is possible to obtain a light-emitting elementfrom which white light is emitted from the whole light-emitting element.Here, the complementary colors refer to colors that can produce anachromatic color when they are mixed. Further, the same can be appliedto a light-emitting element having three EL layers. For example, thelight-emitting element as a whole can provide white light emission whenthe emission color of the first EL layer is red, the emission color ofthe second EL layer is green, and the emission color of the third ELlayer is blue.

Note that the structure described in this embodiment can be combined asappropriate with any structure described in the other embodiments.

Embodiment 6

In this embodiment, an example of application of the lighting devicedescribed in the above embodiment is described.

In FIG. 15, a desk lamp is illustrated as an example of application ofthe lighting device described in the above embodiment. The desk lamp ofFIG. 15 includes a housing 1500 and a light source 1501. The lightingdevice described in the above embodiment is used as the light source1501 in the desk lamp. According to an embodiment of the disclosedinvention, the desk lamp with excellent characteristics can be providedat low cost.

In FIG. 16, an indoor lighting device 1600 is illustrated as an exampleof application of the lighting device described in the above embodiment.The lighting device relating to an embodiment of the disclosed inventionhas a sufficient irradiation range and therefore, is suitable for anindoor lighting device. According to an embodiment of the disclosedinvention, the lighting device with excellent characteristics can beprovided at low cost.

Note that the structure described in this embodiment can be combined asappropriate with any structure described in the other embodiments.

This application is based on Japanese Patent Application serial no.2009-215436 filed with Japan Patent Office on Sep. 17, 2009, the entirecontents of which are hereby incorporated by reference.

1-3. (canceled)
 4. A lighting device according comprising: alight-transmitting base; a first light-transmitting electrode over asurface of the light-transmitting base; an EL layer over the firstlight-transmitting electrode; and a second electrode over the EL layer,wherein the light-transmitting base has a cylindrical shape, wherein athickness of the light-transmitting base over a length of a diameter ofa bottom surface of the light-transmitting base is represented by thefollowing formula:h≧(−n+2.34)×l, wherein h is the thickness of the light-transmittingbase, wherein n is a refractive index of the light-transmitting base,and wherein l is the length of the diameter of the bottom surface of thelight-transmitting base, and wherein l is 10 mm or more. 5-7. (canceled)8. A lighting device according comprising: a light-transmittingsubstrate; a first light-transmitting electrode over a first surface ofthe light-transmitting substrate; an EL layer over the firstlight-transmitting electrode; a second electrode over the EL layer; anda light-transmitting base attached to a second surface of thelight-transmitting substrate which is opposite to the first surface ofthe light-transmitting substrate, wherein the light-transmitting basehas a cylindrical shape, wherein a thickness of the light-transmittingbase over a length of a diameter of a bottom surface of thelight-transmitting base is represented by the following formula:h≧(−n+2.34)×l, wherein h is the thickness of the light-transmittingbase, wherein n is a refractive index of the light-transmitting base,and wherein l is the length of the diameter of the bottom surface of thelight-transmitting base, and wherein l is 10 mm or more. 9-12.(canceled)
 13. The lighting device according to claim 4, wherein thelight-transmitting base includes a glass or a plastic.
 14. The lightingdevice according to claim 8, wherein the light-transmitting substrateincludes a glass or a plastic.
 15. The lighting device according toclaim 4, wherein the EL layer comprises a plurality of light-emittingunits which are stacked.
 16. The lighting device according to claim 8,wherein the EL layer comprises a plurality of light-emitting units whichare stacked.
 17. The lighting device according to claim 4, wherein thesecond electrode is a second light-transmitting electrode.
 18. Thelighting device according to claim 8, wherein the second electrode is asecond light-transmitting electrode.
 19. The lighting device accordingto claim 4, wherein the EL layer includes a transition metal oxide. 20.The lighting device according to claim 8, wherein the EL layer includesa transition metal oxide.